1. Field of the Invention
The present invention relates to a gain control circuit for controlling fluctuations of gain.
2. Description of Related Art
In a variable gain amplifier used for a radio communication system, it is desired that a gain obtained in the variable gain amplifier is exponentially changed with respect to a gain control voltage. Therefore, a conventional variable gain amplifier shown in FIG. 6 is, for example, used.
FIG. 6 is a constitutional view showing a conventional variable gain amplifier. In FIG. 6, 11 indicates a first voltage-to-current converter, 12 indicates a temperature characteristic compensating circuit, 13 indicates an exponentially-changing current producing circuit, 14 indicates a variable gain cell, 15 indicates a second voltage-to-current converter, and 16 indicates a temperature-proportional bias current generating circuit. A combination of the temperature characteristic compensating circuit 12 and the exponentially-changing current producing circuit 13 functions as a gain control circuit, and the exponentially-changing current producing circuit 13 has a differential amplifier 13a. 
Next, an operation of the conventional variable gain amplifier will be described below.
Referring to FIG. 6, a gain control voltage Vcont is applied to the first voltage-to-current converter 11, and the gain control voltage Vcont is converted into a gain control current proportional to the gain control voltage Vcont. Here, a value of the gain control current is expressed by K1xc2x7Vcontxc2x7K1 denotes a proportional constant. Thereafter, the gain control current (K1xc2x7Vcont) is fed to the temperature characteristic compensating circuit 12 according to a current mirror. Also, a bias current proportional to an absolute temperature T is generated in the temperature-proportional bias current generating circuit 16, and the bias current is fed to the temperature characteristic compensating circuit 12 according to a current mirror. The value of the bias current is expressed by K2xc2x7T, and K2 denotes a proportional constant.
Also, a reference voltage Vref fixed with respect to temperature is applied to the second voltage-to-current converter 15, and a gain reference current corresponding to the reference voltage Vref is produced. The value of the gain reference current is constant and is expressed by K3. The gain reference current of the value K3 is fed to the temperature characteristic compensating circuit 12 according to a current mirror.
As is described above, the gain control current (K1xc2x7Vcont), the bias current (K2xc2x7T) and the gain reference current (K3) are respectively fed to the temperature characteristic compensating circuit 12 according to the current mirrors. This type of temperature characteristic compensating circuit 12 is equivalent to a temperature characteristic compensating circuit having a plurality of current sources generating the gain control current (K1xc2x7Vcont), the bias current (K2xc2x7T) and the gain reference current (K3) respectively.
As shown in FIG. 6, the temperature characteristic compensating circuit 12 is composed of a group of a first transistor Q1, a second transistor Q2, a third transistor Q3, a fourth transistor Q4 and a group of a first current source 12a, a second current source 12b, a third current source 12c and a fourth current source 12d. Each of the transistors Q1 to Q4 is formed of an n-p-n transistor (or a first conductive type transistor). A differential amplifier is formed of both the second and third transistors Q2 and Q3. Here, the gain control current (K1xc2x7Vcont) is generated in each of the first current source 12a and the fourth current source 12d, the gain reference current (K3) is generated in the second current source 12b, and the bias current (K2xc2x7T) is generated in the third current source 12c. 
In the temperature characteristic compensating circuit 12, a base of the first transistor Q1 is connected to both a collector and a base of the fourth transistor Q4, and the first current source 12a is connected to the base of the first transistor Q1. The fourth current source 12d is connected to both an emitter of the fourth transistor Q4 and a base of the third transistor Q3. The second current source 12b is connected to both an emitter of the first transistor Q1 and a base of the second transistor Q2. The third current source 12c is connected to both an emitter of the second transistor Q2 and an emitter of the third transistor Q3. A first output current of a value IL is output from a collector of the second transistor Q2, and a second output current of a value IR is output from a collector of the third transistor Q3.
Therefore, the value IL of the first output current is expressed according to an equation (1).
IL=K1xc3x97K2xc3x97Txc3x97Vcont/K3 xe2x80x83xe2x80x83(1)
Also, the value IR of the second output current is expressed according to an equation (2).
IR=K2xc3x97Txe2x88x92(K1xc3x97K2xc3x97Txc3x97Vcont/K3) xe2x80x83xe2x80x83(2)
The first output current of the value IL is fed to the exponentially-changing current producing circuit 13 according to a current mirror. Also, the bias current (K2xc2x7T) is fed to the exponentially-changing current producing circuit 13 according to a current mirror (not shown). Therefore, the exponentially-changing current producing circuit 13 has current sources generating the first output current (IL) and the bias current (K2xc2x7T) respectively. The exponentially-changing current producing circuit 13 is composed of a differential amplifier 13a, a fifth current source 13b, a sixth current source 13c, a seventh current source 13d, a first resistor having a value R1 and a second resistor having the value R1. The differential amplifier 13a is composed of a pair of fifth transistor Q5 and sixth transistor Q6. Each of the transistors Q5 and Q6 is formed of an n-p-n transistor (or a first conductive type transistor). A base voltage is applied to the base of the fifth transistor Q5 through the first resistor, and a base voltage is applied to the base of the sixth transistor Q6 through the second resistor. The bias current (K2xc2x7T) is generated in each of the fifth current source 13b and the sixth current source 13c, and the first output current (IL) is generated in the seventh current source 13d. The bias current (K2xc2x7T) generated in the fifth current source 13b is called a reference current of a value Ia, the bias current (K2xc2x7T) generated in the sixth current source 13c is called a fixed current of a value Ie, and the first output current (IL) generated in the seventh current source 13d is called a control current of a value Ic.
The fifth current source 13b of the reference current (Ia) is connected to a base of the fifth transistor Q5, the sixth current source 13c of the fixed current (Ie) is connected to both an emitter of the fifth transistor Q5 and an emitter of the sixth transistor Q6, and the seventh current source 13d of the control current (Ic) is connected to the base of the sixth transistor Q6. Also, the first resistor (R1) is connected to the base of the fifth transistor Q5, and the second resistor (R1) is connected to the base of the sixth transistor Q6. A third output current having a value Io is output from a collector of the fifth transistor Q5.
Because the value Ic of the control current is equal to the value IL of the first output current, the value Ic of the control current is expressed according to an equation (3) with reference to the equation (1).                                                         Ic              =                              xe2x80x83                            ⁢                                                K                  1                                xc3x97                                  K                  2                                xc3x97                T                xc3x97                                                      V                    cont                                    /                                      K                    3                                                                                                                          =                              xe2x80x83                            ⁢                                                K                  2                                xc3x97                T                xc3x97                                  K                  4                                xc3x97                                  V                  cont                                                                                        (        3        )            
Here, K4=K1/K3 is satisfied.
As shown in FIG. 6, in the exponentially-changing current producing circuit 13, a voltage proportional to a difference between the control current Ic and the reference current Ia is applied to the differential amplifier 13a as an input voltage. In short, a voltage having a value R1(Icxe2x88x92Ia) is applied to the differential amplifier 13a as an input voltage.
Therefore, the value Io of the third output current is expressed according to an equation (4) with reference to the equation (3).                                                                         I                c                            =                              xe2x80x83                            ⁢                              Ie                /                                  [                                      1                    +                                          exp                      ⁢                                              {                                                                                                            R                              1                                                        ⁡                                                          (                                                              Ic                                -                                Ia                                                            )                                                                                /                                                      V                            T                                                                          }                                                                              ]                                                                                                        =                              xe2x80x83                            ⁢                              Ie                /                                  [                                      1                    +                                          exp                      ⁢                                              {                                                                                                            R                              1                                                        ⁡                                                          (                                                                                                                                    K                                    2                                                                    xc3x97                                  T                                  xc3x97                                                                      K                                    4                                                                    xc3x97                                                                      V                                    cont                                                                                                  -                                Ia                                                            )                                                                                /                                                      V                            T                                                                          }                                                                              ]                                                                                        (        4        )            
Here, VT denotes a thermal voltage of a value kT/qxc2x7k denotes a Boltzmann""s constant, and q denotes an elementary electric charge. The thermal voltage is almost equal to 25 mV at ordinary temperature.
As is apparent in the equation (4), in cases where the gain control voltage (Vcont) is low, the third output current (Io) has an exponential function characteristic so as to be exponentially changed with respect to the gain control voltage (Vcont)
The third output current (Io) is fed to the variable gain cell 14 according to a current mirror. Therefore, the variable gain cell 14 has a current source 14a in which the third output current (Io) is generated. The variable gain cell 14 has a differential amplifier 14b composed of a seventh transistor Q7 and an eighth transistor Q8. Each of the transistors Q7 and Q8 is formed of an n-p-n transistor (or a first conductive type transistor). The current source 14a of the third output current (Io) is connected to both an emitter of the seventh transistor Q7 and an emitter of the eighth transistor Q8.
When an alternating current input signal (ACinput) is fed to a base of the seventh transistor Q7 and a base of the eighth transistor Q8, an alternating current output signal (ACoutput) is output from a collector of the seventh transistor Q7 and a collector of the eighth transistor Q8. In this case, a gain (or a level ratio of the alternating current output signal to the alternating current input signal) in the variable gain cell 14 is proportional to the third output current (Io), and the third output current (Io) has the exponential function characteristic with respect to the gain control voltage (Vcont) in cases where the gain control voltage (Vcont) is low. Therefore, the gain in the variable gain cell 14 has the exponential function characteristic so as to be exponentially changed with respect to the gain control voltage (Vcont). In other words, in cases where the gain control voltage (Vcont) is low, a gain of the conventional variable gain amplifier shown in FIG. 6 has the exponential function characteristic so as to be exponentially changed with respect to the gain control voltage (Vcont).
FIG. 7 is a constitutional view showing another conventional variable gain amplifier. The constituent elements, which are the same as those shown in FIG. 6, are indicated by the same reference numerals as those of the constituent elements shown in FIG. 6, and additional description of those constituent elements is omitted.
In FIG. 7, 17 indicates an exponentially-changing current producing circuit of which the configuration differs from that of the exponentially-changing current producing circuit 13. Also, the variable gain cell 14 has a constant current source 14c in place of the current source 14a. 
The exponentially-changing current producing circuit 17 has a pair of differential amplifiers 17a and 17b. The differential amplifier 17a is composed of a ninth transistor Q9 and a tenth transistor Q10, and the differential amplifier 17b is composed of an eleventh transistor Q11 and a twelfth transistor Q12. Each of the transistors Q9 to Q12 is formed of an n-p-n transistor (or a first conductive type transistor). The fifth current source 13b of the reference current (Ia) is connected to both a base of the tenth transistor Q10 and a base of the eleventh transistor Q11, and the seventh current source 13d of the control current (Ic) is connected to both a base of the ninth transistor Q9 and a base of the twelfth transistor Q12. Also, an emitter of the ninth transistor Q9 and an emitter of the tenth transistor Q10 are respectively connected to a collector of the seventh transistor Q7 of the variable gain cell 14, and an emitter of the eleventh transistor Q11 and an emitter of the twelfth transistor Q12 are respectively connected to a collector of the eighth transistor Q8 of the variable gain cell 14. Also, a collector of the tenth transistor Q10 is connected to a first load resistor having a resistance value RL, and a collector of the eleventh transistor Q11 is connected to a second load resistor having the resistance value RL. Also, a base voltage is applied to the bases of the ninth transistors Q9 and Q12 through a resistor having a resistance value R1.
As is described with reference to FIG. 6, the alternating current input signal (ACinput) is fed to the variable gain cell 14. Also, the alternating current output signal (ACoutput) is output from a collector of the tenth transistor Q10 and a collector of the eleventh transistor Q11. A gain Av of the variable gain cell 14 is indicated by a level ratio |ACoutput|/|ACinput|. The gain Av is expressed according to an equation (5).                                                         Av              =                              xe2x80x83                            ⁢                                                g                  m                                xc3x97                                  R                  L                                xc3x97                                  [                                      1                    /                                          {                                              1                        +                                                  exp                          ⁡                                                      (                                                                                                                            R                                  1                                                                ⁡                                                                  (                                                                      Ic                                    -                                    Ia                                                                    )                                                                                            /                                                              V                                T                                                                                      )                                                                                              }                                                        ]                                                                                                        =                              xe2x80x83                            ⁢                                                g                  m                                xc3x97                                  R                  L                                xc3x97                                  [                                      1                    /                                          {                                              1                        +                                                  exp                          ⁡                                                      (                                                                                                                            R                                  1                                                                ⁡                                                                  (                                                                                                                                                    K                                        2                                                                            xc3x97                                      T                                      xc3x97                                                                              K                                        4                                                                            xc3x97                                                                              V                                        cont                                                                                                              -                                    Ia                                                                    )                                                                                            /                                                              V                                T                                                                                      )                                                                                              }                                                        ]                                                                                        (        5        )            
Here, gm denotes a transconductance of the differential amplifier 14b, and RL denotes a resistance value of each load resistor.
Therefore, in the conventional variable gain amplifier shown in FIG. 7, in cases where the gain control voltage (Vcont) is low, the gain of the variable gain cell 14 has an exponential function characteristic so as to be exponentially changed with respect to the gain control voltage (Vcont).
However, because the conventional variable gain amplifier shown in FIG. 6 and the conventional variable gain amplifier shown in FIG. 7 respectively have the above-described configuration, in cases where fluctuations occur in the reference current Ia generated by the fifth current source 13b, a problem has arisen that the gain of each conventional variable gain amplifier is considerably changed. For example, with reference to the equation (4), the third output current (Io) is exponentially changed with the reference current Ia. Therefore, as shown in FIG. 8, in cases where fluctuations occur in the reference current Ia generated by the fifth current source 13b, the third output current (Io) is exponentially changed regardless of the gain control voltage (Vcont) in the conventional variable gain amplifier shown in FIG. 6. Therefore, in cases where fluctuations occur in the reference current Ia, the gain of the conventional variable gain amplifier shown in FIG. 6 is exponentially changed regardless of the gain control voltage (Vcont). In contrast, because the sixth current source 13c of the fixed current (Ie) functions as a fixed current source of the differential amplifier 13a, as is apparent in the equation (4), the third output current (Io) is proportional to the fixed current (Ie). Therefore, even though fluctuations occur in the fixed current (Ie) generated in the sixth current source 13c, the third output current (Io) is merely changed in proportion to the fixed current (Ie). Therefore, even though fluctuations occur in the fixed current (Ie), the gain of the conventional variable gain amplifier shown in FIG. 6 is not changed so much.
In the same manner, in cases where fluctuations occur in the reference current Ia of the fifth current source 13b in the conventional variable gain amplifier shown in FIG. 7, the gain of the conventional variable gain amplifier shown in FIG. 7 is exponentially changed regardless of the gain control voltage (Vcont).
Therefore, in cases where fluctuations occur in the reference current Ia, a problem has arisen that the gain of each conventional variable gain amplifier is considerably and exponentially changed.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional variable gain amplifiers, a gain control circuit which receives no influence of fluctuations of a reference current.
The object is achieved by the provision of a gain control circuit including exponentially-changing current producing means, in which a first differential amplifier composed of a pair of transistors is arranged and bases of the transistors are connected to each other through a base resistor, for receiving a control current of a value K2xc3x97Txe2x88x92(K1xc3x97K2xc3x97Txc3x97Vcont/K3) based on a gain control current (K1xc2x7Vcont) a gain reference current (K3) and a bias current (K2xc2x7T) at the base of one transistor of the first differential amplifier, producing an exponentially-changing current corresponding to the control current and controlling a gain of a variable gain cell according to the exponentially-changing current.
Therefore, a reference current is not required. Accordingly, no influence of fluctuations of a reference current is received in the gain control circuit.